SANTIAGO —
Workers began a surprise strike at the world's largest copper mine, Chile's Escondida, on Wednesday morning to demand improved working conditions and pay.

The work stoppage at Escondida, controlled by global miner BHP Billiton, was initially planned to last 24 hours, but the timeframe will be evaluated in the coming hours, union representative Marcelo Tapia told Reuters.

"[All operations] have been halted," said Tapia, adding the union's 2,500 workers had dropped their tools. "It's due to a variety of issues that the company has not been responding to."

He added that workers at BHP's Spence and Cerro Colorado mines also joined the labor action, which kicked off at 8 am local time (1200GMT).

BHP and Rio Tinto, which own 30 percent of Escondida, declined to comment.

Escondida produced 1.1 million tons of copper last year, about 20 percent of the output from No. 1 copper producer Chile.

The mine's union stunned the copper market in 2011 by staging a two-week work stoppage, sending the mine's output tumbling.

Workers clinched a new contract agreement in January, at the time easing fears of labor unrest at the mine.

Labor action has been galvanized in mining powerhouse Chile as unions seek to get their issues heard ahead of the November presidential election.